This invention relates to novel reactive diluents for use with bismaleimide formulatioss, and more particularly to ethylenically-unsaturated ethers of alkenyl phenols as novel reactive liquid diluents, and to thermosetting bismaleimide formulations comprising bismaleimides and the novel reactive diluents of this invention. The bismaleimide formulations of this invention, when cured, exhibit improved moisture resistance and are particularly useful in combination with fiber reinforcement for producing heat and moisture resistant composites. The novel reactive diluents of this invention provide bismaleimide formulations having improved miscibility and compatability with conventional modifiers including thermoplastic resins, cyanates and the like, and such formulations have a particularly advantageous range of melt viscosities and reactivities.
High strength, high modulus composites are finding increasing use as structural components for use in aircraft, automotive and sporting goods applications. Typically they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix. Such composites may be conveniently fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partially cured matrix resin. Resin systems comprising an epoxide resin and aromatic amine hardener are often used as the matrix resin component of prepreg because they possess an appropriate balance of properties for this composite fabrication process. Although the resulting composites have high compressive strengths, good fatigue characteristics, and low shrinkage during cure, most epoxy formulations absorb moisture and are not well-suited for use at 270.degree. F. or greater in a moisture-saturated condition.
Composites designed for use at temperatures of 300.degree. F. or higher may employ as the matrix resin a combination of a bismaleimide with one or more coreactants such as polyfunctional amines, epoxides, cyanate resins, or comonomers containing polymerizable ethylenic unsaturation. Compositions based on liquid or low-melting solid coreactants and reactive diluents are particularly useful in the production of prepreg materials. A variety of such bismalemide-based formulations are now known, and a number are available from commercial sources.
The range of reactive diluents suitable for use with bismaleimide resins is rather limited. The use of vinyl ether diluents with bismaleimide resins in forming rapid-cure molding resin formulations is disclosed in U.S. Pat. No. 4,609,705. Such formulations gel and cure very quickly, and prepreg based on such formulations would therefore have a brief processing life or "out time". In U.S. Pat. Nos. 4,644,039 and 4,100,140 there are disclosed bis-unsaturated coreactants including diallyl-substituted bispenol A, diallyl-substituted biphenol, diallyl phthalate, triallyl cyanurates and the like, as well as alkenyl phenols such as 2-methoxy-4-allyl phenol (eugenol), and 2-allyl phenol. Also disclosed are the corresponding alkyl ethers, and particularly the methyl ethers, of these alkenyl phenols. Although the corresponding alkenyl ethers are also suggested in general terms in the disclosure of U.S. Pat. No. 4,100,140, there is no specific disclosure or example of such alkenyl ethers. Bismaleimide compositions incorporating a variety of such alkenyl phenols and their use in preparing composites are also disclosed.
Although other liquid diluents are available, including divinyl benzene, esters of acrylic and methacrylic acids and the like, many are highly volatile, have a noxious odor and may be toxic or strong irritants. Some are only poorly miscible with most bismaleimides, and those having reactive hydroxyl or amino functionality may induce rapid crosslinking and premature gellation of the bismaleimide or interact unfavorably with other components of the formulations.
The range of reactive diluents available for use with bismaleimides is thus rather limited, and coreactants and reactive liquid diluents that increase the options available to the resin formulator are clearly needed. Such diluents, particularly if they offer a range of reactivities and viscosities together wth improved miscibility with thermoplastics and compatibility with other commonly used additives and modifiers, would increase the flexibility needed to provide formulations designed to meet the needs of particular end users. In addition, the industry continues to require materials with the ability to withstand ever more severe environments, including elevated temperatures and exposure to extremes of moisture.